Twin probes expected to reach the moon by New Year's Day could reveal details about the inner structure of the moon. Image courtesy NASA.

A pair of lunar probes that will use gravity to reveal secrets of the moon’s interior should arrive in orbit just in time for the new year, NASA scientists said Wednesday.

The twin GRAIL satellites took nearly four months to reach their destination after being launched aboard the same rocket Sept. 10. They separated shortly after launch, then made their way slowly toward the moon, allowing controllers to use less energy and to prepare the probes for their mission.

Linked by radio waves and trailing one behind the other, the probes will pick up slight bumps in gravity as they pass over mountains, craters and masses beneath the moon’s surface.

The lead probe will speed up as it passes over the mass, changing the distance between the probes before the second catches up; ultraprecise measurements of the distance changes, down to the width of a red blood cell, will be converted into high-resolution maps.

While the near-side of the moon is familiar to everyone, it turns out that much about Earth’s nearest neighbor remains an enigma.

Why is the near side of the moon so different from the far side? The near side is known for its basins flooded long ago with volcanic material, while the far side has much different terrain.

“It seems that the answer is not on the surface,” Maria Zuber of the Massachusetts Institute of Technology, the principal investigator for the project, told reporters Wednesday. “We think the answer is locked in the interior. GRAIL is a journey to the center of the moon.”

False-color terrain map shows differences between the moon's near and far sides. High areas are shown in white and red, low in blue and purple.

And questions persist about how the moon was formed. Scientists say it was likely created when a Mars-sized object crashed into Earth very early in the planet’s history, but the details remain sketchy.

Another theory holds that Earth once had two moons, but after a low-velocity impact, the second moon ended up as mountains on the present moon’s far side.

The probes also could reveal details of the moon’s core.

The arrival of the two probes, one on New Year’s Eve and one on New Year’s Day, will likely keep the team at NASA’s Jet Propulsion Laboratory in Pasadena from taking part in New Year’s celebrations.

“We’re all excited about it,” said David Lehman, GRAIL project manager at JPL. “But we won’t be celebrating until we get GRAIL B into orbit on New Year’s Day.”

Also aboard the spacecraft are cameras, to be operated by students in about 2,100 middle schools across the country. There is room for a few hundred more, NASA officials said; details can be found on the GRAIL MoonKAM web page.

Details gathered by the probes in coming months could revolutionize understanding of Earth’s companion.

“We actually know more about Mars, that’s 100 million miles away, than we do about our moon, less than a quarter million miles away,” Zuber said.

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